In electronics, there is an ongoing need to improve the efficiency and bandwidth of amplifier circuits. For example, in RF and microwave systems, wideband amplifiers are essential components for boosting signals in a wide range of technology, from telecommunications equipment to radar and electronic warfare systems. However, improvements in amplifier bandwidth are often made at the expense of amplifier efficiency, and vice versa.
One method for improving the efficiency of an amplifier was invented in 1934 by William H. Doherty. The so-called Doherty amplifier is shown in FIG. 1. It consists of a class-B main amplifying stage 11 in parallel with a class-C auxiliary amplifier stage 13. There are two quarter-wave transformers, LI and LO, provided at the input of auxiliary amplifier 13 and at the output of main amplifier 11, respectively. The input signal is split evenly to drive the two amplifiers, and a combining network 15 sums output signals from the main and auxiliary stages and corrects for phase differences between them. During periods of low signal level, the main stage efficiently amplifies the input signal and the auxiliary stage remains off. During high power signal peaks, the main stage saturates and the auxiliary stage turns on. This increases overall efficiency dynamic range (which is the input power range over which efficiency remains high) by about 6 dB. While the Doherty amplifier is effective in boosting the efficiency dynamic range, it is not well suited for wideband applications due to inherent bandwidth limitations.
A well-known wideband amplifier known as a distributed amplifier (DA) was invented by William S. Percival in 1936. DA architecture introduces delay to achieve wideband characteristics. One model of a DA is shown in FIG. 2. The DA consists of a pair of transmission lines (an input line 17 and an output line 19) with characteristic impedances ZI (1 to 5) and ZO (1 to 5) independently connecting the inputs and outputs of several active devices, as shown. In this example, the active devices are modeled as field effect transistors (FETs) Q1, Q2, Q3, Q4. As an input signal propagates down the input line 17, the individual FETs respond to the forward-traveling input step by inducing an amplified forward-traveling wave on the output line 19. The gain of a DA is additive rather than multiplicative, and is determined in part by the number of stages. This property enables the DA to provide gain at frequencies beyond that of the unity-gain frequency of any individual stage. The delays of the input and output lines can be made equal through selection of propagation constants and line lengths to ensure that the output signals from each individual device sum in phase. However, a major drawback to the DA is poor efficiency, because power matching and phasing cannot be achieved at the same time. As a result, much of the output power is wasted in the termination load 21.
The foregoing discussion illustrates the tradeoff of efficiency for bandwidth in the design of electronic amplifiers, What is needed to meet industrial demand is a highly efficient wideband amplifier.